CODEWITHSUNDEEP
c++arraysvectorstl
Taher Lunawadi
Taher Lunawadi

Reputation: 76

How to fix 'invalid operands to binary expression'?

I wrote the following program in C++ and it turned up the following errors. It probably seems to be a problem with traversing the 2D vector

I have already tried out writing int j = matrix.at(0).begin() but that doesn't work as well

I have come across the following errors

 invalid operands to binary expression ('vector<int>::iterator' (aka '__wrap_iter<int *>') and 'std::__1::vector<int, std::__1::allocator<int> >::size_type' (aka 'unsigned long'))


no viable conversion from 'std::__1::vector<std::__1::vector<int, std::__1::allocator<int> >, std::__1::allocator<std::__1::vector<int, std::__1::allocator<int> > > >::iterator' (aka '__wrap_iter<std::__1::vector<int, std::__1::allocator<int> > *>') to 'int'

#include <stdio.h>
#include <vector>
#include <iterator>
using namespace std;

int diagonalSum(vector<vector<int> > matrix) {
    int sum[2] = {0};
    vector<int> s;
    for(int i = matrix.begin();i < matrix.size();i++)
    {
        for(int j = matrix[i].begin();j < matrix.[i].size();j++)
        {
            if(i == j)
                sum[0]+=matrix[i][j];
            if((i+j) == matrix.size())
                sum[1]+=matrix[i][j];
        }
    }
        return (sum[0] + sum[1]);
}
int main()
{
    vector <vector<int> > matrix = {
            {2,4,6},{4,8,12},{6,12,18}
    };
    int dSum = diagonalSum(matrix);
    return 0;
}

Upvotes: 1

Views: 3631

Answers (2)

Michael Veksler
Michael Veksler

Reputation: 8475

This code has several problems.

First, performance:

int diagonalSum(vector<vector<int> > matrix) {

The above copies the matrix with all its elements every time you call the function, for no apparent reason. Complexity: O(n2).

Better pass by reference to const (complexity O(1)):

int diagonalSum(const vector<vector<int> > & matrix) {

Then the code tries to copy an iterator object, from matrix.begin(), which is supposed to iterate over objects of type vector<int>, and tries to assign this iterator into an integer variable. The integer and the iterator are of incompatible types, and can't be assigned to one another, and hence the error.

I assume you wanted to use an index instead:

for(int i = 0;i < matrix.size();i++)
{
    for(int j = 0; j < matrix[i].size();j++)

And even that is a bad solution, from a complexity stand-point. Your solution is O(n2), where your task can be easily performed in O(n) with one loop.

Upvotes: 3

Mike
Mike

Reputation: 46

Vector.begin() method returns an iterator pointing to the first element in the vector. That's why you can't assign it to int variable.

Change your for loop to this

for(int i = 0;i < matrix.size();i++)
    {
        for(int j = 0;j < matrix[i].size();j++)
        {
            if(i == j)
                sum[0]+=matrix[i][j];
            if((i+j) == (matrix.size() -1))
                sum[1]+=matrix[i][j];
        }
    }

Upvotes: 2

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